1. Technical Field
The invention relates to a method for continuous plasma treatment of workpieces. In particular the invention relates to a method of continuous plasma treatment of boards or sheets of material at atmospheric pressure. A plasma treatment in the sense of this invention is understood in particular to refer to a plasma coating.
2. Background Information
In the finishing industry or in the production of plastic films, the latter are activated at the surface by a plasma treatment at atmospheric pressure, also known as a corona treatment. An industrial corona system usually consists of a high-voltage electrode and a counter-electrode designed as a roll and guided over the plastic film, which is in close proximity. The electrode is arranged parallel to the roll, with the electrode being connected to a high voltage of approximately 10 kilovolts at approximately 20-40 kilohertz and the roll being connected to ground potential. Due to the potential difference in the air gap between the high-voltage electrode and the roll with the plastic film, amounting to a few millimeters, a corona discharge develops with conventional practical power outputs of 1 to 5 kilowatts per meter. The plastic film is activated by the corona discharge, i.e., oxidized at the surface.
Due to this activation, the surface tension is increased to thereby ensure adequate adhesion of printing inks and adhesives.
For higher adhesion demands, a primer must be applied to the plastic film after the plasma activation. Primers generally consist mostly of solvent plus a component that improves adhesion. After application of the primer, e.g., by rolling, the solvent must be removed. This is accomplished through a drying tunnel, and in the case of an organic solvent, it must be burned thermally.
This process is thus associated with high investment costs and the costs of ongoing operation in view of the components required—applicator mechanism, drying and afterburning.
Therefore, there have already been attempts for many years to replace wet chemical primer application with a plasma coating process that is performed at atmospheric pressure and does not require a solvent. This eliminates the drying and afterburning. The applicator is then replaced by an atmospheric pressure plasma coating unit.
To do so, a carrier gas, e.g., air, nitrogen, a noble gas or mixtures thereof with precursors, e.g., tetramethyl orthosilicate or hexamethylene disiloxane, is fed into the atmospheric pressure plasma discharge in the electrode system to coat the substrate, which is arranged on the grounded counter-electrode.
In addition to the substrate, the electrode system itself is also coated here. Therefore, the required discharge gap between the electrode system and the counter-electrode is closed and the coating process must be interrupted for cleaning, which is not acceptable for continuous industrial processes.